1. Technical Field
The present invention relates to methods of manufacturing composite substrates for epitaxial-layer growth, formed by bonding together a first substrate composed of a nitride compound semiconductor, and a second substrate composed of a material that is either identical with or different from the material constituting the first substrate.
2. Background Art
Semiconductor microelectronic devices, such as light-emitting devices, (also denoted simply “devices” hereinafter) employing nitride compound semiconductors are created by a variety of methods. In general, they are fabricated by forming a nitride compound semiconductor layer, by molecular-beam epitaxy (MBE), metal organic vapor phase epitaxy (MOCVD) or a like technique, onto a substrate composed of a material (such as silicon, silicon carbide or sapphire, for example) that is of a different type from, yet whose thermal expansion coefficient is close to, the nitride compound semiconductor, or by forming nitride-compound semiconductor layers onto a nitride compound semiconductor bulk substrate. Either way, fabricating the devices by growing epitaxial layers onto substrates is the general practice.
When a substrate of a material that differs from the nitride compound semiconductor is used to fabricate devices, disparity in thermal expansion coefficient and lattice mismatch will arise. This leads to the problems of stress developing in the substrate, which gives rise to deterioration in the device properties due to warpage in the substrate and devices, and of exfoliation of the semiconductor layers and increase in dislocation density arising, which exerts a negative influence on the device properties.
Meanwhile, when nitride compound semiconductor bulk substrates are employed, because the thermal expansion coefficient and the lattice-matching between the bulk substrate and the semiconductor layers are on par, favorable device properties are exhibited. Nevertheless, a problem with nitride compound semiconductor bulk substrates is that they extraordinarily expensive, such that the cost is reflected in the price of the devices.
Therein, for the reason that despite favorable device properties being obtained when nitride compound semiconductor bulk substrates are employed the cost proves expensive, the development of low-cost substrates for nitride compound semiconductor layer formation (that is, for epitaxial layer growth) is being variously investigated.
For example, reducing the thickness of a bulk substrate constituted from a nitride compound semiconductor may lower the cost of the substrate, yet disadvantageously leaves it prone to fracturing. Consequently, attention is being given to composite substrates obtained by joining thinly to the nitride compound semiconductor a second substrate made of a different substance from the nitride compound semiconductor. Technology for fabricating a composite substrate is disclosed for example in Patent Literature 1.
With the composite substrate fabrication method of Japanese Unexamined Pat. App. Pub. No. 2006-210660, a fragile region is formed in advance within the nitride compound semiconductor for ease of exfoliation, and the nitride compound semiconductor in which the fragile region has been formed is bonded together with a second substrate. Then, after the nitride compound semiconductor and the second substrate are unified, with the fragile region as a boundary, the nitride compound semiconductor is severed from the second substrate. Forming a thin nitride compound semiconductor in this way atop the second substrate fabricates a composite substrate. In the following, the technology for composite substrates fabricated in this way will be denoted “bonding technology” in particular.